Chadcoleman created wonderful tutorials, how to added Custom Size Post Thumbnail. All we need to do only put some functions and use it whatever and whenever you want.

Custom Size Thumbnail - Avril Lavigne

Added this code on your functions.php

if ( function_exists( 'add_theme_support' ) ) { // Added in 2.9
	add_theme_support( 'post-thumbnails' );
	set_post_thumbnail_size( 50, 50, true ); // Normal post thumbnails
	add_image_size( 'my-custom-size', 405, 180, true );
}

That it’s, you have new custom size for you thumbnail and you can call it with Post Thumbnail Tricks